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How to Understand Diagnostic Trouble Codes (DTCs) for Commercial Trucks

Learn about how to understand diagnostic trouble codes.

Episode 215: You’re driving down the highway, and then suddenly, a light appears on your dash. That’s an indicator that maybe you’ve got some trouble coming. In today’s episode, we focus on diagnostic trouble codes, how to interpret them, what they mean, and how to go from a trouble code to the part you need in under 3 minutes.

Links Mentioned in Episode:

Diesel Repair logo. In this episode Learn about how to understand diagnostic trouble codes.

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Transcript of Episode:

Jamie Irvine:

You’re listening to The Heavy-Duty Parts Report. I’m your host, Jamie Irvine. And this is the show where you get expert advice about heavy-duty parts that keeps trucks and trailers on the road longer while lowering cost-per-mile.

You’re driving down the highway and all of a sudden a light appears on your dash. And that’s an indicator that maybe you got some trouble coming. So in today’s episode, we would like to focus on diagnostic trouble codes, also called diagnostic fault codes. We wanna help you understand how to interpret them what they mean and how we can go from a diagnostic trouble code all the way to the part that you need in under three minutes, let’s get started.

So our friends over at Diesel Laptops recently published a blog article that talks about understanding diagnostic trouble codes. Also the acronym for that is DTC. So you might see that. And they put together some really great information that will help us really understand what they are, how to interpret them and what to do with that information and how to access that the information you need to repair your commercial truck and get the part that you need to get that truck back on the road.

So, as I said, diagnostic trouble codes are also known as DTCs and they are an alert from your truck’s electronic control module. Also known as an ECM, and it’s an alert that an issue exists. And when a DTC becomes active, this indicator shows up on your dash, like I said, as a light to make sure the driver is aware that, Hey, there is a problem. The light can look different depending on which system is triggering the fault code.

So if you have your ABS braking system versus your diesel emission system or something related to your engine or transmission, you might see a different light in a different area on the module. Now in today’s modern commercial trucks, there are thousands of possible diagnostic fault codes that could occur. And as I mentioned, they can be displayed in many ways. The first is in your display.

However, these codes often contain many acronyms like SA, MID, SPN, FMI, and there’s other jargon. Now we’re gonna drop a link in the show notes to this article. And in there, you’re going to see that there is also a link to an article that was put out by Diesel Laptops, where they actually interpret all of that commercial jargon. They put it all in a convenient blog post.

So we’ll make sure that there’s a link to the blog article about diagnostic fault codes. And we’ll also make sure in the show notes that there is a link to the article about commercial truck jargon, so that you will be able to decipher what those codes actually mean. But those letters, those acronyms they’re the short version. And obviously that’s so that, that doesn’t take up too much space on the actual display.

Now, another tool using the electronic logging devices or ELDs found in many over-the-road trucks, you can view the diagnostic fault codes on your mobile device or through a website platform through your ELD.

And these have the same issues as your in-cab display though and sometimes it’s even worse than that, where the information that that’s giving you is not gonna be all that helpful, or it might even give you the information, but is it in a format that you can interpret? And some of the protocols are proprietary to the truck manufacturers. So then the ELD devices are not designed really for diagnostics.

So they really shouldn’t be treated as such. So even though you might be able to get some of that information through your ELD, it’s not really recommended. Instead, you know, the folks at Diesel Laptops, they really encourage, especially repair technicians to use a proper diagnostic tool to get the information.

But what if you’re not a repair technician and you are not really in the market for a $12,000 laptop? Well, there is a great product from Diesel Laptops called Diesel Decoder, and that is something that is cost-effective and will give you some basic information. It’s perfect for drivers, or do it yourself mechanics, or also parts technicians who want to be able to quickly plug in and help the customer figure out what’s going on there.

Okay. So let’s talk about taking the fault code and going all the way to getting the part you need to repair the truck, because this is the big vision that diesel laptops, right? They want to go from DTC to repair and part in under three minutes and give you every bit of information you need. And they’re doing that with a platform that they’re developing. So once you have your diagnostic fault code, you can use an app called Diesel Repair to find out information about that fault code.

This information can include repair information, labor time guides, there’s a bunch of other information in there that you might need, but once you have that fault code, you can put it into the Diesel Repair app that’s something that you can load on your Android device, your Apple device, that can be on your desktop computer as well. And you’re gonna receive a bunch of information. You’re gonna get an overview of the fault code, the component on the vehicle causing the problem.

It’s gonna tell you what that is, the symptoms the driver may observe. So you can compare what did the drivers say to what the information on this fault code does that match up the conditions that need to be met for the fault code to be diagnosed? So there’s some information there that you need, also systems of the vehicle affected because that’s something that’s really important.

You have to remember, a commercial truck is just one huge system made up of a bunch of other smaller systems. And oftentimes just because you have one fault code and it’s pointing at one part, it doesn’t mean you don’t have a problem elsewhere, upstream causing that.

Perfect example is in your diesel emission system, your DPF filters constantly being plugged. It’s not the DPF filter, that’s the problem, right? Something’s causing that filter to get plugged up. Maybe it’s a turbo, maybe it’s somewhere something else in the system. So looking at the whole system and getting the information you need to identify the systems connected to that is very important. You’re gonna get that with Diesel Repair.

You’re also gonna get information on how to reset the fault code once the repairs are completed. And it’ll also give you the probable causes of repair or the probable things you would need to look at to make that repair.

So this is all the kinds of great information you get from the Diesel Repair app that you can download for free. One note about active fault codes is if you ignore them, sometimes some of those fault codes will go inactive, but typically you’re gonna need to run a reset command to get that fault code off. We’re gonna take a quick break. We’ll be right back.

Don’t have a heavy-duty part number and need to look up a part? Go to parts.diesellaptops.com or download the app on Apple or Android to create your free account.

Looking for high-quality fuel injection for heavy-duty applications? Having one supplier for fuel injection allows you to better serve customers by providing them with a complete line, which increases your sales and profitability. Learn more at ambacinternational.com/aftermarket.

Now, let’s talk a little bit about OBD2 and J1939, how to read those trouble codes and what the information in those two different protocols looks like.

So, first of all, let’s talk about OBD2. It’s a five-digit code, right? The first character indicates where the problem is occurring, and those digits are displayed as P powertrain, C chassis, B body, or U network. And that could include all onboard computer systems.

So I’ll just go through that again. So P for powertrain, this is the engine, transmission and fuel systems. The C for chassis includes steering suspension and brakes, B for body include systems inside the vehicle up to, and including AC it’s summertime, you’re gonna need that. And U network all onboard computer systems.

The second digit of the DTC indicates whether or not the code is a standard or manufacturer-specific code. The standard digit displayed is usually either zero or one. However, on rare occasions, the number might be two or three. Okay. So zero indicates a standard code across all vehicles.

One, two or three indicates the manufacturer’s specific code with two or three displaying, relatively rare occurrences. Okay. So what does that mean? So if you have a zero this is something that could affect any vehicle. If you have a one it’s probably something that’s specific to the manufacturer of that component or system on the truck, the third digit displayed out of the five digits indicates a subsystem.

So now we’re starting to drill down from these main categories into other categories. One is usually a fuel or air metering issue, two is can also be fuel or air metering issues. So depending on whether it’s in the five-digit code is a one or a two it’s the same system, but it just could be pointing at different problems within that system. Three is usually ignition four is the emission system, five vehicle control or speed ,six the computer circuitry, seven transmission, eight also can be transmission.

The fourth and fifth characters are read together as a two-digit number, somewhere between zero and 99. And these are known as the specific fault code index. These characters identify the exact issue of the vehicle, and you’re really gonna need to check with your vehicle manufacturer or even vehicle manual for explanations of these codes.

So an example of this type of code. So again, we’re talking the OBD2 DTC code, an example could be P0101, and here would be a list of common fault codes. So P0101 is a mass airflow circuit or sensor fault P 0110 is an intake or air temperature sensor circuit malfunction P0442, a small system leak within the vehicles of evaporative emission control system. Another code you might see is P0500 a malfunction in the vehicle speed sensor, P0606.

That could be a PCM or ECM powertrain malfunction and P0706 is a transmission range, sensor circuit range fault. So, um, that’s how those OBD two fault codes work that’s at what they look like. Let’s move on to J 1939 DTCs.

So these codes are four-digit. Each digit representing a system that points you toward a specific problem for your vehicle. Okay, so this includes suspect parameter number or SPN. It represents the overall system affected by the malfunction, which corresponds to a standard set of numbers.

The failure mode identifier, the FMI represents the nature and type of the error. The next digit in this code is the occurrence counter adds up. The number of times this error occurs for every SPN and the SPN conversion methods. The acronym for that is CM represents the bite alignment for the DTC code.

So structured a little differently, but same basic way or I would say architecture for drilling down on a specific fault code. So what you want with all this information, whether you’re a driver, you’re a repair technician, you’re parts technician, you wanna be able to figure out like a step by step repair information guide to go from, okay, we have this fault code all the way through to what part do I need to buy and how do I fix it? And that’s what Diesel Repair is all about.

Embedded inside of Diesel Repair, in addition to all that repair information, you also get access to Diesel Parts and Diesel Parts is a parts lookup tool. That goes way beyond just a cross-reference. It’s got year, make model lookups. It’s got by measurement tools, it’s got component searches, reverse lookups, what does it fit?

It’s an amazing tool embedded in diesel repair. Now you can go and download diesel repair for free, and you get 30 days to try it out for free before you have to pay their monthly subscription. But here is a little hack. If you’re just looking for parts information, I’m gonna drop the link for Diesel Parts, which is incorporated in the Diesel Repair, but it also is a standalone app for parts people. It is 100% free to sign up. And right now it is 100% free to use.

It has been so now for a couple years that looks like that’s going continue for some time. So you have free unfettered access to Diesel Parts. If you want to get into the diesel repair information, after that 30 days, there is a subscription fee, but it is well worth it if you’re a repair technician.

So all the links will be in the show notes of this episode. Thanks for tuning in. And I hope that this clarifies a little bit about the fault codes gives you the information that you need. And again, I’m gonna drop the links in the show notes to the specific article so that you can go back and reread it. If you need to, we’re gonna put, drop a link to the jargon article so it can help you figure out what all those acronyms actually mean. And let’s get our trucks and trailers back on the road. Let’s keep ’em rolling. Let’s lower total cost of operation.

Thank you so much for listening to the heavy-duty parts report. And I don’t normally do this, but I want to call out right now. If you haven’t followed the show. If you’re listening on your favorite podcast app, hit that follow button, give us a review if you can. If you’re on YouTube, subscribe to our channel. And if you are finding us through TNC radio or some other method, go over to our website, heavydutypartsreport.com. You can actually follow there. I’ll send you an email every week with new content. So you never miss out. Thanks so much for your support. We’ll talk to you soon.

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